autonomic nervous system dr. michael p. gillespie

Autonomic Nervous System

Dr. Michael P. Gillespie

Autonomic Nervous SystemAuto = self; nomic = lawThe autonomic nervous system (ANS) includes autonomic

sensory neurons, integrating centers in the central nervous system (CNS), autonomic motor neurons, and the enteric division.

The ANS is also referred to as the visceral nervous system or involuntary nervous system.

It acts as a control system. It operates largely without conscious control; however,

centers in the hypothalamus and brain stem do regulate ANS reflexes.

Dr. Michael P. Gillespie 2

Flow of nerve impulses in the ANS(1) Autonomic sensory neurons in visceral

organs and blood vessels propagate into (2) integrating centers in the CNS. Then, impulses in (3) autonomic motor neurons propagate to various effector tissues, thereby regulating the activity of smooth muscle, cardiac muscle, and many glands. (4) The enteric division is a specialized network of nerves and ganglia forming an independent nerve network within the wall of the gastrointestinal tract.


Functions of ANSThe ANS affects heart rate, digestion,

respiratory rate, salivation, perspiration, pupillary dilation, micturition (urination), and sexual arousal.

Most autonomous functions are involuntary.Some ANS actions can work with some

degree of conscious control:BreathingSwallowingSexual arousalHeart rate


Divisions of the ANSThe ANS is classically divided into two

subsystems: the parasympathetic nervous system (PSNS) and sympathetic nervous system (SNS).

For some functions these systems operate independently and for others they operate co-operatively.

In many cases the PSNS and SNS have opposite actions with one activating a physiological response and the other inhibiting it.

The enteric nervous system is also considered to be a part of the ANS.


Afferent and EfferentANS functions can be divided into sensory

(afferent) and motor (efferent) subsystems.


Somatic Nervous System The somatic nervous system includes both sensory and motor neurons. Sensory neurons convey input from receptors for somatic senses (tactile,

thermal, pain, proprioceptive) and special senses (sight, hearing, taste, smell, equilibrium).

These sensations are normally consciously perceived. Somatic motor neurons innervate skeletal muscles to produce both voluntary

and involuntary movements. When a somatic motor neuron stimulates a muscle, it contracts; the effect is

excitation. If it fails to stimulate a muscle it becomes paralyzed. A few skeletal muscles, such as those in the middle ear, are controlled by

reflexes and cannot be contracted voluntarily.


Autonomic Sensory NeuronsThe main input to the ANS comes from autonomic

(visceral) sensory neurons. These neurons are primarily associated with interoceptors (monitor the internal environment).

Most of the time, these signals are not consciously perceived; although, intense activation of interoceptors may produce conscious sensation.

Pain sensations from damaged viscera and angina pectoris (chest pain) form inadequate blood flow to the heart can produce conscious sensation.


Autonomic Motor NeuronsAutonomic motor neurons regulate visceral activities by

either increasing (exciting) or decreasing (inhibiting) activities in the effector tissues (cardiac muscle, smooth muscle, and glands).

Examples of autonomic motor responses include: changes in the diameter of the pupils, dilation and constriction of blood vessels, adjustment of the rate and force of the heartbeat.

Unlike skeletal muscle, tissue innervated by the ANS often continue to function to some extent even if their nerve supply is damaged.

The heart continues to beat when it is removed for transplantation, smooth muscle lining the GI tract contracts rhythmically on its own, and glands produce some secretions in the absence of ANS control.


Two Divisions of ANS OutputUnlike the somatic output (motor), the output

portion of the ANS has two divisions: sympathetic division and parasympathetic division.

Most organs have dual innervation (both PSNS and SNS innervation).

In some organs, nerve impulses from one division stimulate the organ to increase its activity (excitation) and impulses from the other division decrease its activity (inhibition).


Sympathetic Nervous SystemThe sympathetic division is often called the fight-or-flight

division.Sympathetic activities result in increased alertness and

metabolic activities in order to prepare the body for an emergency situation.

Physical activity and emotional stress can trigger sympathetic activities.

Effects of sympathetic stimulation: rapid heart rate, faster breathing rate, dilation of pupils, dry mouth, sweaty but cool skin, dilation to blood vessels to organs involved in combating stress (heart and skeletal muscles), constriction of blood vessels to organs not involved in combating stress (GI tract and kidneys), and release of glucose from the liver).


Parasympathetic DivisionThe parasympathetic division is often

referred to as the rest-and-digest division because its activities conserve and restore body energy during times of rest or digesting a meal.

The majority of the nerve output is directed towards smooth muscle and glandular tissue of the gastrointestinal and respiratory tracts.

The parasympathetic division conserves energy and replenishes nutrients.


Comparison of Somatic and Autonomic Nervous Systems


Two-neuron Efferent PathwayThe ANS differs from the somatic nervous system in that it

requires a sequential two-neuron efferent pathway.The preganglionic neuron will synapse with a

postganglionic neuron before innervating the target organ.The first of the two motor neurons is called the

preganglionic neuron. The cell body is located in the brain or spinal cord. It exits the CNS as part of a cranial or spinal nerve.

It synapses with a postganglionic neuron in an autonomic ganglion, which is the second neuron in the autonomic pathway. The postganglionic neuron is located entirely in the PNS.


Preganglionic NeuronSympathetic Division

Thoracolumbar division [cell bodies in the lateral horns of the 12 thoracic segments and the first two (sometimes three) lumbar segments]

Thoracolumbar outflow [axons]Parasympathetic Division

Craniosacral division [cell bodies in the nuclei of four cranial nerves (III, VII, IX, & X) and the lateral gray matter of the second through fourth sacral segments]

Craniosacral outflow [axons]


Structure of the Sympathetic Division


Structure of the Parasympathetic Division


Adrenergic and Cholinergic Receptors


Autonomic GangliaSympathetic gangliaParasympathetic ganglia


Sympathetic GangliaSympathetic ganglia – sites of synapse between

sympathetic preganglionic and postganglionic neurons.Sympathetic trunk ganglia (also called vertebral chain

ganglia or paravertebral ganglia). Lie in a vertical row on either side of the vertebral column. Extend from the base of the skull to the coccyx. Innervate organs above the diaphragm (head, neck and heart). Superior, middle, and inferior cervical ganglia.

Prevertebral (collateral) ganglia. Lie anterior to the vertebral column. Innervate organs below the diaphragm. Celiac ganglion, superior mesenteric ganglion, inferior

mesenteric ganglion, aorticorenal ganglion, and renal ganglion.


Parasympathetic GangliaParasympathetic ganglia – sites of synapse

between preganglionic parasympathetic neurons and postganglionic neurons in the terminal (intramural) ganglia.

Terminal ganglia in the head have specific names: ciliary ganglion, pterygopalatine ganglion, submandibular ganglion, and otic ganglion.

The remaining terminal ganglia do not have specific names.


Postganglionic NeuronAxons of preganglionic neurons pass to sympathetic trunk

ganglia and synapse with postganglionic neurons.A single sympathetic preganglionic fiber has many axon

collaterals and may synapse with 20 or more postganglionic neurons. This divergence explains why many sympathetic responses affect almost the entire body simultaneously.

After exiting their ganglia, the postganglionic axons typically terminate in several visceral effectors.

Axons of preganglionic neurons of the parasympathetic division pass to terminal ganglia near or within a visceral effector.


Comparison of Sympathetic And Parasympathetic Divisions of ANS


Effects of Sympathetic And Parasympathetic Divisions of ANS


Autonomic Plexuses Thorax

Cardiac plexus – innervates the heart Pulmonary plexus – innervates the bronchial tree

Abdomen and Pelvis Celiac (solar) plexus [largest plexus]– innervates the

stomach, spleen, pancreas, liver, gallbladder, kidneys, adrenal medulla, testes, & ovaries

Superior mesenteric plexus – innervates the small and large intestines

Inferior mesenteric plexus – innervates the large intestine Hypogastric plexus – innervates the pelvic viscera Renal plexus – innervates the renal arteries within the

kidneys and ureters


Postganglionic Neurons in the Sympathetic Division


Autonomic Plexuses in the Thorax, Abdomen, and Pelvis


Sympathetic Trunk GangliaCervical PortionThoracic PortionLumbar PortionSacral Portion


Pathways from Sympathetic Ganglia to Visceral Effectors Spinal nerves (31 pairs) – innervate visceral effectors of the skin of

the neck, trunk, and limbs including sweat glands, smooth muscle in blood vessels, arrector pili muscles. Gray rami communicantes – structures containing sympathetic

postganglionic axons that connect the ganglia of the sympathetic trunk to the spinal nerves.

Cephalic periarterial nerves – innervate visceral effectors of the skin of the face (sweat glands, smooth muscle of blood vessels, arrector pili muscles. Innervates visceral effectors of the head (smooth muscle of the eye, lacrimal glands, pineal gland, nasal mucosa, and salivary glands.

Sympathetic nerves – innervates visceral effectors in the thoracic cavity including the heart and lungs.

Splanchnic nerves – innervates the organs of the abdominopelvic cavity.


Sacral Parasympathetic Outflow


Parasympathetic Division of the ANSPreganglionic neurons originate from the

cranial nerves III, VII, IX, & X as well as the sacral spinal nerves S2-4.

The presynaptic neuron usually synapses with 4-5 postsynaptic neurons, all of which supple a single visceral effector.


Enteric DivisionPlexuses

Myenteric plexusSubmucosal plexusThis system controls motility and secretory

functions of the gastrointestinal tract.


Cholinergic NeuronsCholinergic neurons – acetylcholine AchCholinergic neurons include:

All sympathetic and parasympathetic preganglionic neurons

Sympathetic postganglionic neurons that innervate most sweat glands

All parasympathetic postganglionic neurons


Cholinergic ReceptorsCholinergic receptors bind with acetylcholine Two types

Nicotinic receptorsMuscarinic receptors


Adrenergic Neurons and ReceptorsRelease noripinephrine (noradrenalin)Most sympathetic postganglionic neurons are

adrenergic.Two types:

Alpha receptorsBeta receptors


Cholinergic and Adrenergic Neurons in the Autonomic Nervous System


Autonomic ToneAutonomic tone is a balance between

sympathetic and parasympathetic activity.Autonomic tone is regulated by the

hypothalamus.


Sympathetic Responses Stress ↑sympathetic system ↑fight-or-flight response. ↑ production of ATP Dilation of pupils ↑heart rate and blood pressure Dilation of airways Constriction of blood vessels that supply the kidneys and

gastrointestinal tract ↑blood supply to the skeletal muscles, cardiac muscle, liver and

adipose tissue ↑glycogenolysis ↑blood glucose ↑lipolysis


Parasympathetic ResponsesRest-and-digest responseConserve and restore body energy↑digestive and urinary function↓Body functions that support physical

activity


Integration and Control of Autonomic FunctionsDirect innervation – brain stem and spinal

cordHypothalamus is the major control and

integration center of the ANSIt receives input from the limbic system


autonomic nervous system dr. michael p. gillespie

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